Structure–properties relationship in the hydronium-containing pyrochlores (H3O)1+pSb1+pTe1−pO6 with catalytic activity in the fructose dehydration reaction

• Published in: Dalton transactions : an international journal of inorganic chemistry Vol. 49; no. 33; pp. 11657 - 11667
• Main Authors: Mayer, Sergio Federico, Falcón, Horacio, Fernández-Díaz, María Teresa, José Miguel Campos-Martín, Alonso, José Antonio
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 25.08.2020
• Subjects: Antimony; Apexes; Basicity; Catalytic activity; Crystal structure; Dehydration; Fructose; Hydroxymethylfurfural; Inductively coupled plasma; Ion exchange; Optical emission spectroscopy; Pyrochlores; Spectrum analysis; Structural analysis; Sulfuric acid; Tellurium
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d0dt01770a

A series of defect pyrochlores of the composition (H3O)1+pSb1+pTe1−pO6 have been prepared by ion exchange from K-containing pyrochlores K1+pSb1+pTe1−pO6 in sulfuric acid at 280 °C for 24 h. The structural characterization of the hydronium-containing pyrochlores, including the location of the H3O+ units within the three-dimensional framework, was possible from neutron powder diffraction data in undeuterated samples. The crystal structure for all the compounds is defined in the Fd3m space group, and consists of a covalent framework of SbVO6 and TeVIO6 octahedra distributed at random and connected by their vertices with (Sb,Te)–O1–(Sb,Te) angles close to 136°, conforming to large cages where the hydronium species are located off-center. The absence of K+ ions in the ion-exchanged pyrochlores was confirmed by inductively coupled plasma optical emission spectroscopy and scanning electron microscopy coupled with energy dispersive X-ray spectroscopy. The shape and size of the hydronium units evolve along with the series, becoming more compact as the framework covalence and Lewis-basicity decrease upon Sb enrichment of the structure (for greater p values). The amount and lability of the H3O+ species also increase throughout the series, as wanted: a straightforward correlation of the catalytic activity in the fructose dehydration reaction to 5-hydroxymethylfurfural has been observed, reaching conversion rates up to 88.5% of concentrated fructose solution for the p = 0.25 catalyst. Moreover, a pseudo-first-order kinetic mechanism was simulated, and the kinetic constants obtained from diluted and concentrated enhanced reaction systems were determined and compared.